NSW Police are searching the Tweed River for a man who may have fallen in while walking beside the waterway.

Search teams are combing the Tweed River in northern NSW for a man aged in his 60s who may have fallen in.

NSW Police say a witness saw a man, dressed in a white T-shirt, grey trackpants and joggers, walking along the riverbank about 10pm (AEDT) on Monday.

The witness heard a splash and could no longer see the man.

Police started the search on Monday night for a man they believe may be called John, and the search has continued on Tuesday bringing in Queensland Police and SES volunteers.